When reservoir compaction occurs during production, the overburden formation deformation results in ocean floor subsidence and lateral movement. When a long term water or CO2 injection operation is performed, the overburden formation becomes unstable due to the reservoir expansion. These field operations induce formation strain and pore pressure change in all overburden formations. Predicting the magnitude of formation strain and pore pressure change is important for analysis of borehole stability, casing stability and overburden formation stability.
Two methods are developed in this paper. One is an analytical method assuming no mechanical property contrast between overburden and reservoir formations. The other is a set of figures created by a numerical model for predicting formation strains and pore pressure change during compaction and expansion with mechanical property contrast between overburden and reservoir formations. The parameter analyses give the following results:
Formation strain becomes significantly large if the Young's modulus of overburden formation becomes small. The casing design must be conducted including the formation strain of cap rock.
If the shear modulus between overburden and reservoir formations is similar, the pore pressure increases at the overburden formations. However, if the difference of the shear modulus is large, the pore pressure decreases in the upper and lower cap rock with reservoir compaction. This new finding may change our common sense of pore pressure change of shale section around a reservoir.
This paper provides a set of equations and figures to quickly predict the pore pressure change and formation strain focusing on overburden formation where casing and formation stability problems are induced during production and injection operations. If the quick analysis indicates that the overburden strain and pore pressure change significantly affect the field operations, comprehensive analysis must be conducted with a geomechanical finite element model using a super computer.